Diesel engines typically tend to be larger, heavier and more massive than comparable gasoline engines. This is because of the heavier construction required to withstand rates of combustion pressure rise immediately after ignition and the higher combustion pressures needed for compression ignition. In particular, usually a major portion of the combustion chamber is located in the piston. This generally causes the engine to be taller and also leads to the requirement for a heavier stronger piston.
Diesel engines are also known for some unburned hydrocarbon exhaust emissions that can result in increased deposits, especially on the piston and elsewhere in the combustion chamber. This creates difficulties with very small displacement engines where the clearance between the piston and cylinder head is small.
Diesel engines have, however, certain advantages that make them suitable for applications where durability is important. Unfortunately, the bulkiness of diesel engines has hitherto been a major disadvantage in mobile applications, especially when they have to be delivered to site by aircraft or human transport. The key to reducing the weight of the engine is to reduce the rates of combustion pressure rise in conjunction with lower compression ratios. Unfortunately, with these lower compression ratios it is hard to get diesel fuels to ignite by compression ignition.